Being as you have started with very simple questions, I will give you a few very simple pieces of advice. Its free so don't complain.
First make sure you want and need 20 MOA bases. I am planning on trying a 350-450yd assault with a 22 rimfire on a local acquaintance of mine in the next four or five days now that the weather is warm and he is poking his head out. I checked my dialup and can marginally get to about 350 yds without any adjustment to the bases.
Second, depending on which gun it is and you are a little skimpy on critical information in that category, You can just shim and bed the base you have if it is one piece.
Third, until you have a scope mounted you may not be able to tell the alignment of the top of the receiver in relation to the barrel. It is entirely possible that your gun left the factory with some receiver slope that either helps or hinders.
Fourth, be careful and don't get the gun set up for five hundred yds and not be able to dial back do to 25 yds.
There is probably a fifth and sixth but I will let you have some fun on your own.